The present invention concerns a method of producing viaholes in glass fiber reinforced plastic sheets and an application of that method for fabricating plastic sheets with unilaterally or bilaterally applied conductor patterns and viaconnectors conductively linked to the conductor patterns.
The method is used in particular to fabricate multilayer boards. Such boards are laminates of several glass fiber reinforced plastic sheets, to every other one of which a conductor pattern is unilaterally or bilaterally applied. The conductor patterns are conductively interconnected by copper layers on the walls of viaholes in the plastic sheets. The methods suitable for fabricating multilayer boards include, for instance, the so-called "pin-parallel method". This method is described in the article "High-density board fabrication techniques" by J. R. Bub et al published in the IBM Journal of Research and Development, Vol. 26, No. 3, May 1982, p. 306 and subsequent pages. By this method a copper layer is bilaterally applied to a plastic sheet. Subsequently, holes in accordance with the desired placement of the viaholes are drilled into the copper layers and the plastic sheet either mechanically or by a laser beam. The holes are cleaned and activated for electroless plating by means of a solution produced from PdCl.sub.2 and SnCl.sub.2. The two copper layers are subsequently covered with a photoresist mask, representing the negative of the desired conductor pattern, and finally copper is deposited in the holes and on the bared regions of the copper layers. Then the photoresist masks and the bared regions of the thin copper layer are removed. From the plastic sheets thus obtained a package is formed, alternately using untreated plastic sheets, and laminated. Finally, throughholes are drilled into the package and, after activation (see above) their walls are electrolessly copper-plated. During the formation of the holes with a diameter of about 0.25 mm in the about 0.15 mm thick and about 70.times.60 cm large sheets and also in particular during the cleaning of the holes, for example, by sandblasting, the plastic sheets are subjected to high thermal and/or mechanical stresses. The holes have a very small spacing, so that close tolerances have to be observed during drilling or laser beam treatment, which is difficult to accomplish by the tools described. Apart from this, the drilling or laser beam irradiation step is time-consuming and expensive from an apparatus point of view. Furthermore, the mechanical stability of the plastic sheets is greatly reduced during the formation of the holes, because the glass fiber meshing in the holes is removed in the same process. Finally, the conductive connectors in the holes formed by the described process fall short of the optimum values, since the copper coating is thin and not continuous throughout.